1. Field of the Invention
The present invention relates to the field of directional couplers. Particularly the present invention relates to hybrid directional couplers and its application to multi-input/multi-output signal wave guides.
2. Art Background
A directional coupler is a four-port wave guide in which an incoming wave at any one port appears at the 2 adjacent ports but not at the non-adjacent fourth port. This device is employed in wave guide networks such as microwave wave guides, integrated optics, and optical fibers.
A simple example of directional coupling is shown with reference to FIG. 1. Two conductors 10, 20 are oriented side by side over a conducting plane. The current I in conductor 10 will induce a current I.sub.m in conductor 20 because of magnetic coupling. The actual value of the current will depend on the external circuitry attached to the conductors but it will be assumed that the two of them extend to infinity in both directions. Since capacitive coupling exists also, a second set of current components denoted by I.sub.c will flow. The result is that a wave traveling toward the right in conductor 10 will produce a wave traveling toward the left in conductor 20. Such coupling is called contradirectional coupling since the induced wave travels in the opposite direction to the generating wave.
The sections of transmission lines which are in close proximity to one another function as transformers with the feature that the coupling is directional. An example of using the effective directional coupling is shown by FIG. 2 which shows two sources coupled to the common load without cross-coupling of power from one source to the other. This configuration is referred to as a hybrid combiner or coupler and is often used to combine the outputs of two solid state amplifiers in order to increase the power handling capability. This provides the use of less expensive low power devices.
Referring to FIG. 2, the circuit operates as follows: a wave from a signal generator 40 located at the left end of transmission line 50 travels toward the right and induces a wave on transmission line 60 that travels toward the left and on into the load 70. No wave is induced on line 60 that travels toward the right except for a small fraction of power. A similar situation exists with the second signal generator 80 connected at the right end of transmission line 60. A wave is induced on line 50 that travels toward the right since the load is also connected to the right end of line 50. Power in the induced wave will be dissipated here with little energy reaching the generator 40 at the end of line one.
For further information on directional couplers and hybrid couplers, in particular, see, Radio Amateur Handbook (American Radio Relay League 1989) and McGraw-Hill Encyclopedia of Science and Technology, Vol. 15, pp. 338-340 (6th Edition 1987).
In order to maximize the amount of power coupled and induced on the second signal line and to minimize the negative or destructive effects which may occur, each port of the hybrid device is terminated to a common impedance. By terminating each port to the characteristic impedance of the device, the negative effects of backwards crosstalk caused by reflections generated by a signal traveling between mismatched impedances of media are avoided.